CM Draconis 3

Astronomers have been trying to
observe planetary transit eclipses
across a tight binary of red dwarf
stars in the CM Draconis system.

System Summary

While most astronomical sources
still list a distance from Sol of 47.3 light-years (ly) --
apparently as estimated in the early 1970s -- for the CM Draconis
system, some astronomers studying the star more recently have been
citing a revised distance of 54 to 55 ly away. In any case, this
very dim system can be found in the southcentral part
(16:34:20.2+57:9:46.5, ICRS 2000.0; 16:34:24.5+57:8:58.5,
J2000) of Constellation Draco
(chart,
with
CM
Dra circled at page bottom), the Dragon -- northwest of
Rastaban
(Beta Draconis) and southeast of Eta Draconis. However, none of
the stars in this system of two red and one white dwarf stars are
bright enough to be seen with the naked eye, although the two
close-orbiting red dwarfs are known to be low frequency flare
stars. Astronomers now suspect that the system is around nine
billion years old. (See a CCD field image of
CM
Draconis and comparison stars used in differential photometry
from the Transits of Extrasolar Planets --
TEP --
Network of
astronomers.)

The relatively large proper motion of the CM Draconis system
was discovered in the early 1960s by
Willem
Jacob Luyten (1899-1994), who found the proper motions of
over 520,000 stars despite the loss of sight in one eye since
1925 by building an automated photographic plate scanner and
measuring machine. In 1967, the reddish component was found
to be an eclipsing binary by
Olin
Jeuck Eggen and Allan Sandage. In 1996, two teams of
astronomers announced the possible detection of a
planetary
transit eclipse of the close binary pair CM Draconis Aab,
which has yet to be confirmed (further
details below).

This cool and dim, main sequence red dwarf (M4.5 Ve) and its
companion Ab have a total disc area of about 12 percent that of
Sol's and a combined luminosity of 1.03 percent of Sol's
(Doyle et al,
2000). Star Aa may have about 24 percent of Sol's mass and
25 percent of its diameter (see a TEP page on
CM Draconis).
Its large space velocity of 164 km (102 miles) per second and
low flare rate are consistent with its probable status as a
relatively old, Population II star
(McCook
et al, 1997; and
Metcalfe
et al, 1996). The spectrum of stars Aab is similar to that of
Barnard's Star
(S.
M. Rucinski, 1978).

CM Draconis Aa has a close, double-lined spectroscopic companion
Ab, with which it forms a non-contact, eclipsing binary, and a
wide common-proper-motion companion B. Stars Aab and B have an
observed separation of about 420 AUs (25.7"). On the other hand,
stars Aa and Ab have are separated by only 3.76 solar radii, which
is about 14.9 times the radius of CM Draconis Aa
(Claud
H. Lacy, 1977), and have a mutual orbital period of just
under 1.27 days
(Deeg et
al, 1998;
Metcalfe
et al, 1996; and
Claud
H. Lacy, 1977). According to a TEP page on
CM Draconis,
their close orbit is high circular (e= 0.0050 +/- 0.0015) and
inclined by 89.8° from the perspective of an observer on Earth.
(See an animation of the
orbits of the two stars and their
potentially habitable zone, with a table of basic orbital and
physical characteristics.)

CM Draconis Aab may both be flare stars, like much
younger UV Ceti (Luyten 726-8 B)
shown flaring at left.
UV Ceti is an extreme example of a flare star that can
boost its brightness by five times in less than a minute,
then fall somewhat slower back down to normal
luminosity within two or three minutes before flaring
suddenly again after several hours.

CM Draconis Ab (or "B" or "C")

This main sequence red dwarf star (M4.5 V) may have only 21
percent of Sol's mass and 6.2 percent of its diameter
(see TEP page on
CM
Draconis).

CM Draconis B is a white dwarf (a remnant stellar core which
enriched its binary companions, Stars Aab,
with elements heavier than hydrogen when it cast off its outer gas
layers) like planetary nebula NGC
2440.

CM Draconis B (or "C")

This even dimmer, main sequence white dwarf star (DQ8 /VII) may
have only 18/100,000th of Sol's luminosity. The stellar remnant
has cooled, become dimmer over time, and is presumed to be
relatively old (perhaps around nine billion years like its
binary companions). In theory, it has around 0.6 to 1.4 times
Sol's mass with only a planet-sized diameter (e.g., around one
percent of the Sun's). Useful catalogue numbers include:
CM Dra B, GJ 630.1 B, G 226-15, G 225-68, EG 258, LHS 422, and
LP 101-16.

View from an asteroid of tightly orbiting red dwarfs with a
tidally-locked planet in the system's liquid water zone. The
hypothetical planet's darkside ice cap is backlit from a
nearby white dwarf in the distance.

In 1996, two teams of astronomers announced the possible
detection of a
planetary
transit eclipse of the close binary pair CM Draconis Aab
(Guinan
et al, 1998;
Martin
and Deeg, 1996; and
Guinan
et al, 1996). The initial discovery of a brown-dwarf-sized object
was disputed
(Deeg
et al, 1998). Eventually, it was determined that no substellar
objects around CM Draconis larger than about three Jupiter-masses
are orbiting closer than one AU
(Doyle et al,
2000). As of mid 2000, further observations and analysis
suggested that one (or two) planet(s) of at least 2.5 Earth
diameters across in a 21- or a 26-day orbit and a farther out
giant planet in the range of 1.5 to three Jupiter masses at an
orbital distance of 1.1 to 1.45 AU around the close-orbiting
binary's barycenter
(Deeg
et al, 2000).

Many dim, red (M) dwarf stars exhibit unusually violent flare activity
for their size and brightness. These flare stars are actually common
because red dwarfs make up more than half of all stars in our galaxy.
Although flares do occur on our Sun every so often, the amount of
energy released in a solar flare is small compared to the total amount
of energy that Sol produces. However, a flare the size of a solar
flare occurring on a red dwarf star (CM Draconis) that
is more than ten thousand times dimmer than our Sun would emit about
as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares spaced anywhere
from an hour to a few days apart. A flare only takes a few minutes to
reach peak brightness, and more than one flare can occur at a time.
Moreover, in addition to bursts of light and radio waves, flares on dim
red dwarfs may emit up to 10,000 times as many X-rays as a
comparably-sized solar flare on our own Sun, and so flares would be
lethal to Earth-type life on planets near the flare star. Hence,
Earth-type life around flare stars may be unlikely because their
planets must be located very close to dim red dwarfs to be warmed
sufficiently by star light to have liquid water (just over 0.1 AU for
CM Draconis Aab), which makes flares even more dangerous around such
stars. In any case, the light emitted by red dwarfs may be too red in
color for Earth-type plant life to perform photosynthesis efficiently.

Closest Neighbors

The following star systems are
located within 10 ly of CM Draconis, assuming a distance from Sol of
47.3 ly. (Unfortunately, this system is too far away to be included
in ChView's Nearby Star Map and too dim
for its Bright Star Map.)

Constellation Draco is associated with the dragon slain by Cadmus,
the brother of Europa. It is a large and elongated constellation
of the northern hemisphere and is one of the few constellations
which really resemble the object they were named after.
For more information about the stars and objects in this
constellation and an illustration, go to Christine Kronberg's
Draco.
For another illustration, see David Haworth's
Draco.

For more information about stars including spectral and luminosity
class codes, go to ChView's webpage on
The Stars of
the Milky Way.